Wideband detector for use in coaxial transmission lines

ABSTRACT

Coaxial line apparatus forming a termination and detector for high frequency signals. An input coaxial transmission line has an inner conductor and an outer conductor. The input coaxial transmission line is branched into a load transmission line and a parallel video transmission line. The load transmission line has inner and outer conductors and a matching impedance connected between the inner and outer conductors for terminating the load transmission line. The parallel transmission line has a separate outer conductor and inner conductive means. The inner conductive means include a crystal diode mounted at an angle with respect to the axis of the parallel transmission line outer conductor. The parallel transmission line outer conductor has an apertured portion substantially aligned with the crystal diode so that the crystal diode can be inserted or removed through the apertured portion. The parallel transmission line also includes additional impedances for attenuating relatively high frequency signals.

United States Patent Mouw [54] WIDEBAND DETECTOR FOR USE IN COAXIAL TRANSMISSION LINES [72] inventor: Robert B. Mouw, Menlo Park, Calif.

[73] Assignee: Aertecb, Sunnyvale, Calif.

[22] Filed: Aug. 6, 1971 [21] Appl. No.: 169,648

[52] U.S. Cl. ..329/162, 329/205 R, 333/73 C, 333/82 B, 333/96 [51] Int. Cl. ..H03d 9/02 [58] Field of Searcb.. ...329/162, 205 R, 203; 333/96, 333/73 C, 82 B, 81 A; 325/445, 446

Primary Examiner-Alfred L. Brody Attorney-Flehr, Hohbach, Text, Albritton & Herbert a Sept. 19, 1972 [57] ABSTRACT Coaxial line apparatus forming a termination and detector for high frequency signals. An input coaxial transmission line has an inner conductor and an outer conductor. The input coaxial transmission line is branched into a load transmission line and a parallel video transmission line. The load transmission line has inner and outer conductors and a matching impedance connected between the inner and. outer conductors for terminating the load transmission line. The parallel transmission line has a separate outer conductor and inner conductive means. The inner conductive means include a crystal diode mounted at an angle with respect to the axis of the parallel transmission line outer conductor. The parallel transmission line outer conductor has an apertured portion substantially aligned with the crystal diode so that the crystal diode can be inserted or removed through the apertured portion. The parallel transmission line also includes additional impedances for attenuating relatively high frequency signals.

15 Claims, 7 Drawing Figures PATENTED EH I912 I 3.693.103

SHEET 2 0F 2 firm/mini WIDEBANI) DETECTOR FOR USE IN COAXIAL TRANSMISSION LINES BACKGROUND OF THE INVENTION This invention generally relates to apparatus for use in high frequency systems of the coaxial type and more particularly pertains to a wideband coaxial termination and high impedance detector.

Whenever a detector is used to monitor or measure microwave energy it is desirable that its impedance be matched to that of the coaxial system with which it is associated. This is necessary in order to obtain a true indication of the power in the system. When there is a large mismatch, there are large standing waves in the system and the indication of energy is erroneous. It is also desirable to obtain a constant sensitivity over a broad band of frequencies while maintaining a low voltage standing wave ratio (VSWR).

The prior art includes several examples of termination and detector arrangements including matching impedances for terminating coaxial lines. These prior an systems generally use a coaxial arrangement of a detecting element transmission line and the terminating resistance in which the detecting element is located inside the termination resistance and by-passed to ground. With this arrangement the high impedance detector (i.e., crystal diode) is contained in the middle of the coaxial apparatus. Therefore, every time a detector diode burns out, it is necessary to replace the entire coaxial unit rather than being able to simply replace the diode. Also, it is difficult and expensive to manufacture the coaxially mounted detecting element and termination resistance configuration. There is therefore a need for an improved wideband detector for use in coaxial transmission lines which is not subject to these disadvantages and limitations.

SUMMARY OF THE INVENTION AND OBJECTS It is an object of this invention to provide an improved wideband detector having a load transmission line and a parallel high impedance detector transmission line.

It is an object of this invention to provide a wideband detector in which the high impedance detecting element is easily replaced.

It is another object of this invention to provide a wideband detector having a low VSWR over a wide frequency range.

Briefly, in accordance with one embodiment of the invention, there is provided coaxial line apparatus used as a termination and sensing means for high frequency signals including an input coaxial transmission line having an inner conductor and an outer conductor. Means are provided forming a junction by which the input coaxial transmission line is branched into a transmission line terminating in a matched load and a separate parallel transmission line containing the desired detectmg means.

Other objects and advantages of the invention will appear from the following description of the preferred embodiments given in conjunction with the accompanying drawings.

2 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation partially in cross-section of a coaxial termination and detector in accordance with this invention.

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1.

FIG. 3 is a top plan view taken along the line 3-3 of FIG. 1.

FIG. 4 is a sectional view taken. along the line 4-4 of FIG. 1.

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 2.

FIG. 6 is a sectional view taken along the line 6-6 of FIG. 2.

FIG. 7 is a schematic electrical diagram of a coaxial termination and detector arrangement in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a wideband coaxial termination and detector assembly 11 having an input end 11a and an output end 11b. An input housing 12 is formed of a conductive material and comprises an outer conductor for an input transmission line. A type N nut 13 is rotatably mounted to the input housing 12 by a snap ring 14. A washer l5 surrounds a portion of the input housing 12. The input housing 12 has a threaded portion 12a which engages a threaded portion 16a of main housing 16 for joining the input housing 12 to the main housing 16. The threaded portion 16a of the main housing 16 includes a coaxial recess in which an insulating member 17 is mounted. The insulating member 17 may be, for example, a Teflon bead. A Teflon bead 17 supports a center conductor 18 which has one end terminating in a pin 19 adjacent to input end 11a of the assembly 11. The Teflon head 17 has undercut portions and 17b. These undercut portions serve to introduce inductance into the input transmission line which compensates for capacitance associated with steps in the outer conductor of the input transmission line which includes part of the input housing 12 and the threaded portion 16a of main housing 16.

An output housing 21 is also provided which threadedly engages and is held by the main housing 16. A video output connector 22 is threadedly mounted in the output housing 21 and terminates in an outer conductor 23 surrounding an inner conductor 24.

The center conductor 18 of the input transmission line extends through the Teflon head 17 into a space interior of the main housing 16. The center conductor 18 and the input housing 12 cooperate to form a coaxial input transmission line. In the interior of the main housing 16 the coaxial input transmission line is branched into two separate transmission lines. Integral with the center conductor 18 is a center conductor section 26 forming a junction means for branching the input transmission line into two separate transmission lines. A top plan view of the center conductor section 26 is shown in FIG. 3. The center conductor section 26 has undercut portions 26a and 26b and has a central opening generally indicated by reference numeral 27. The purpose of the undercut portions 26a and 26b is to introduce inductance into the circuit to compensate for capacitive effects associated with the detector 38. The center conductor section 26 also has an end portion 28 for engaging and making electrical contact to a terminal 29 of a resistor assembly 31. The resistor assembly 31 has an additional terminal 32 which terminates in and is held by a conductive button 33. The terminal 32 extends through a central opening of the conductive button 33 and makes electrical contacts therewith. The conductive button 33 is secured at its periphery by suitable means such as by soldering to the main housing 16.

The resistor assembly 31 in accordance with one embodiment of the invention is a rod resistor having an impedance value corresponding to the characteristic impedance of the input transmission line, which may be, for example, 50 ohms. A suitable rod resistance is a FILMOI-IM No. 6123 rod resistor.

The main housing 16 has a chamber 34 which retains a sensing element or detector assembly 36. A portion of the chamber 34 is threaded and is adapted to receive a set screw 37 for holding the detector assembly 36 in place. The detector assembly 36 includes a detector 38 which may be, for example, a crystal diode having one terminal 38a adapted to extend into the central opening 27 in the center conductor section 26 for making electrical contact therewith. The detector 38 also has an additional terminal 38b and the detector assembly 36 includes means such as detector cap 39 for making electrical contact with the additional terminal 38b. Disposed in the chamber 34 and surrounding a portion of the detector assembly 36 is a body of lossy material 41. One lossy material suitable for the body of lossy material 41 is Emerson and Cummings Eccosorb MFl24. The body of lossy material 41 equalizes the response of the detector 38 to the high frequency input signals. In other words, without the lossy material around the detector 38, the detectors response would rise to a peak of approximately 6 dB above its average response over a band of frequencies. By utilizing the lossy material, whose impedance increases with frequency, the detectors response is equalized so that it is flat over a wide frequency range.

The branched parallel transmission line containing the sensing means or detector assembly 38 also includes a resistor assembly 42 mounted in the main housing 16 forming part of an inner conductive means. A portion of the main housing 16 surrounds the parallel transmission line inner conductive means to form a transmission line completely separate from the load transmission line. Resistor assembly 42 has a terminal 43 extending into the detector cap 39 and making electrical contact therewith. The resistor assembly 42 also has an additional terminal formed of a bellows assembly 44 which is adapted to make electrical contact with a video contact 46. An elevation of video contact 46 is shown in FIG. 4. The video contact 46 comprises a body of insulating material 47 which may be glass epoxy, for example, having a conductive coating 48 on one face thereof. A portion of the conductive coating 48 is removed so that two concentric conducting rings 48a and 48b are formed. The bellows 44 contacts and makes electrical connection with the outer conducting ring 48a. The chip resistor 49 typically has an impedance of 470 ohms and its purpose is to eliminate output voltage discontinuities that might occur if low frequency energy leaked into the parallel video transmission line.

The main housing 16 has a threaded aperture generally indicated by reference numeral 53 in which a set screw 54 is disposed. The set screw 54 is adjustable within the threaded aperture 53 and may be adjusted so as to extend up into a chamber 56 formed in the interior of the main housing 16 for affecting the electrical characteristics thereof. That is, as discussed before, there are capacitive effects associated with the detecting means or diode 38. The center conductor section 26 is undercut on either side of the opening 27 in order to introduce inductance for compensating for the capacitive effects associated with the detector 38. The center conductor section is undercut more adjacent the end portion 28 (undercut section 26b) than it is at its other end (undercut section 26a). The center conductor section is undercut to such an extent as to overcompensate for the capacitive effects associated with the detector 38. By adjusting the set screw 54 up into the chamber 56 capacitive effects are added to the characteristics of the load transmission line. In this manner, the capacitive effects of the detector 38 may be exactly compensated without having to worry about critical dimensions in undercutting the center conductor section 26.

As can be seen from FIG. 1, the walls of the chamber 56 in the vicinity of the resistor assembly 31 are stepped. First, second and third steps generally indicated by reference numerals 57, 58 and 59 are shown in FIG. 1. As electromagnetic fields move down the load transmission line, the lossy material on the center conductor, i.e., rod resistor 31, tends to cause the fields to bend forward. That is, electromagnetic fields travel faster near the outer conductor than they do near the inner conductor because the outer conductor is lossless and the inner conductor is lossy. Therefore, the outer conductor has to be reduced in diameter so that the fields terminate on the outer wall normal to the outer wall. A perfect curve for the outer wall so that the field lines are maintained transverse is known as a tractorial curve, which is well known in the art. A tractorial curve is, however, very hard to machine because it is a nonlinear taper. Therefore, the steps 57, 58 and 59 are used to empirically approximate a tractorial curve.

The main housing 16 includes a diode ejection access hole generally indicated by reference numeral 51 which is closed by a removable set screw 52. Thus, a portion of the video transmission line comes off the input transmission line at substantially a right angle. The diode or detecting element is located in this transverse segment so that it is easy to manipulate the diode and to replace it. A termination and detector arrangement such as in the present invention can absorb approximately one watt of RF power. However, a detector such as the diode 38 burns out at about milliwatts, leaving however the basic resistor and capacitor networks intact. Using the present construction wherein an input transmission line is branched into a load transmission line and a video transmission line with the detecting element of the video transmission line located transverse to the input transmission line and load transmission line, the detecting element can be easily replaced without disturbing other elements of the assembly. Thus, to replace the diode or detecting element 38 in applicants assembly, it is only necessary to remove the set screw 37 and the detector cap 39. Then a blunt drift such as a wooden dowel for example, can be inserted through the opening 51 in order to push the detector element or diode 38 out of the detector assembly in chamber 34.

The assembly 11 also includes a video capacitor assembly 61. A cross-sectional view of the video capacitor assembly 61 is shown in FIG. 5. The video capacitor assembly 61 is mounted in the main housing 16 in aperture 62 thereof. The aperture 62 has an extended threaded portion 62a which mounts a set screw 63 for locking in place the detector assembly 36 shown in outline form in FIG. 4. A conductive member 64 is wrapped in an insulating tape 66 which may be, for example, Teflon tape and is disposed in the aperture 62 in the main housing 16. The conductive member 64 has a 67 is fitted in the spring retaining recess 64a. An insulating bushing which may be, for example, a nylon bushing 68 is provided for centering the spring 67 and providing a surface against which the spring 67 acts. The end of the conductive member 64 opposite the insulating bushing 68 is adapted to bear against and make electrical contact with the outer conducting ring 48a on the video contact 46. Spring 67 yieldably urges the conductive member 64 against this outer conducting ring 48a. The video capacitor assembly 61 forms with the resistor assembly 42 an RC circuit for attenuating signals above a predetermined frequency level in the video transmission line.

Referring now to FIG. 6, there is shown a cross-sectional view of the resistor assembly 42 illustrating the details thereof. Terminal 43 extends into a center conductor 69 which has a spring finger portion 71 adapted to engage and make electrical contact with a terminal 72 of a resistor 73. The resistor 73 has an additional terminal 74 connected to the bellows assembly 44. A suitable element for resistor 73 is a 56 watt 68 ohm resistor. A body of lossy material 76 surrounds the center conductor 69 and the body of the resistor 73 to equalize the impedance of the resistor assembly 42 over a range of frequencies.

Referring to FIG. 7, there is shown a schematic electrical diagram of a wideband coaxial termination and detector in accordance with this invention. An input transmission line is branched at a center conductor section 26 into a load line and a video transmission line. The load line terminates in a resistance 31 which has a value corresponding to the characteristic impedance of the input transmission line. The video transmission line includes a sensing element which may be a diode 38 and which is surrounded by lossy material 41 which serves to equalize the diode response over a range of microwave frequencies. A resistor 42 and a video capacitor 61 are provided in the video transmission line and cooperate to form an RC circuit which filters out microwave signals above a certain frequency. According to one embodiment of applicant's invention, signals having a frequency below 1.5 GHZ and above 10 MHz are filtered out by the RC combination of resistor 42 and video capacitor 61. The video transmission line is terminated with a resistance 49 which has a relatively high value (on the order of 470 ohms) to terminate the video transmission line in what is essentially an open circuit for attenuating signals having frequencies below MHz, for example.

I claim:

1. In a high frequency wideband detector for measuring the properties or values of an electromagnetic signal in a coaxial transmission line path, a first coaxial transmission line through which said electromagnetic signal is passed, a load resistance terminating said transmission line and connected between the inner conductor and outer conductor thereof, means serving to match said load resistance to thereby prevent development of standing waves on said coaxial transmission line, means forming a junction on said first transmission line prior to said load resistance, a second coaxial transmission line connected to said junction and formed of inner and outer conductors separate from said inner and outer conductors of said first coaxial transmission line, a detecting element positioned in said second transmission line and connected to the center conductor of said first transmission line, said detecting element having an input impedance substantially higher than said first transmission line impedance whereby said second transmission line has no appreciable effect upon said first transmission line, means associated with said detecting element for maintaining the overall impedance of said detecting element and said last named means at a substantially constant value over a wide frequency range, a video output transmission line connected to the other side of said detecting element and an RF return bypass capacitor connected across the video transmission line to ground.

2. In coaxial line apparatus of the type used as a termination and detector for high frequency signals, an input coaxial transmission line comprising an inner conductor and an outer conductor, means forming a junction so that said input coaxial transmission line branches into a load transmission line and a parallel transmission line, said load transmission line having inner and outer conductors, a matching resistance having a first terminal connected to said inner conductor and a second terminal coupled to said outer conductor of said load transmission line for terminating the same, said parallel transmission line having an outer conductor and inner conductive means, separate from said load transmission line, said parallel transmission line inner conductive means having portions including lossy material and a high impedance detecting element, said parallel transmission line inner conductive means terminating in an inner conductor output portion coaxial with said parallel transmission line outer conductor.

3. Coaxial line apparatus in accordance with claim 2 in which said high impedance detector comprises a crystal diode.

4. Coaxial line apparatus in accordance with claim 3 wherein the portion of said parallel transmission line inner conductive means comprised by said crystal diode forms an angle with respect to the axis of said parallel transmission line outer conductor and wherein said parallel transmission line outer conductor has an apertured portion substantially aligned with said crystal diode whereby said crystal diode can be inserted or removed through said apertured portion.

5. Coaxial line apparatus in accordance with claim 4 in which the angle formed by said crystal diode with respect to the axis of said parallel transmission line outer conductor is substantially a right angle.

6. Coaxial line apparatus in accordance with claim 4 in which the portion of said lossy transmission line inner conductive means comprised by lossy material is also disposed at the same angle with respect to the axis of said lossy transmission line outer conductor as said crystal diode, said lossy material extending into said apertured portion of said outer conductive means and surrounding a portion of said crystal diode, means for adjusting the position of said lossy material with respect to said crystal diode to equalize the response of the crystal diode to the high frequency signals over a wide frequency range.

7. Coaxial line apparatus in accordance with claim 4 wherein said lossy transmission line inner conductive means includes an impedance element connected between said crystal diode and said inner conductor output portion, said impedance element having resistance and capacitance and including a center conductor in series with a lumped resistance and lossy material forming a capacitor bead surrounding said center conductor and lumped resistance.

8. Coaxial line apparatus in accordance with claim 7 including an additional impedance element comprising a capacitance connected in said lossy transmission line between said lumped resistance and said outer conductor, said additional impedance element cooperating with said impedance element to form an RC filter for filtering out signals on said lossy transmission line below a predetermined frequency.

9. Coaxial line apparatus in accordance with claim 8 including a chip resistor having a relatively high resistance connected between said impedance element and said inner conductor output portion for attenuating relatively low frequency signals on said lossy transmission line.

10. Coaxial line apparatus in accordance with claim 2 wherein said matching resistance is a rod resistance forming a continuation of said load transmission line inner conductor and wherein said load transmission line outer conductor has a plurality of discontinuous steps in the vicinity of said matching resistance approximating a tractorial taper to compensate for slow signal propagation through said matching resistance.

11. Coaxial line apparatus in accordance with claim 3 wherein said means forming a junction includes a center conductor section having one end continuous with said input center conductor and having another end connected to said matching resistance first terminal, said center conductor section having an aperture between its ends mating with said crystal diode.

' l2. Coaxial line apparatus in accordance with claim 11 wherein said center conductor section has undercut portions for introducing series inductance on either side of the crystal diode connection to the center conductor section in order to compensate for capacitive effects associated with said crystal diode.

13. Coaxial line apparatus in accordance with claim 12 wherein said center conductor section undercut portions overcompensate for the crystal diode capacitance and including a threaded aperture in said load transmission line outer conductor adjacent the first terminal of said matching resistance and further includin a threaded screw member disposed in said threa ed aperture and threadably ad ustable therem for inserting additional capacitance to bring the effective discontinuity susceptance of said junction means to zero.

14. Coaxial line apparatus in accordance with claim 2 wherein said input transmission line outer conductor has a coaxial recess, an insulating bead mounted in said coaxial recess, said insulating bead having a central opening through which said input transmission line inner conductor passes.

15. Coaxial line apparatus in accordance with claim 14 in which said insulating bead has its central opening counterbored to introduce inductance to compensate for the capacitance introduced by said coaxial recess in said input transmission line outer conductor. 

1. In a high frequency wideband detector for measuring the properties or values of an electromagnetic signal in a coaxial transmission line path, a first coaxial transmission line through which said electromagnetic signal is passed, a load resistance terminating said transmission line and connected between the inner conductor and outer conductor thereof, means serving to match said load resistance to thereby prevent development of standing waves on said coaxial transmission line, means forming a junction on said first transmission line prior to said load resistance, a second coaxial transmission line connected to said junction and formed of inner and outer conductors separate from said inner and outer conductors of said first coaxial transmission line, a detecting element positioned in said second transmission line and connected to the center conductor of said first transmission line, said detecting element having an input impedance substantially higher than said first transmission line impedance whereby said second transmission line has no appreciable effect upon said first transmission line, means associated with said detecting element for maintaining the overall impedance of said detecting element and said last named means at a substantially constant value over a wide frequency range, a video output transmission line connected to the other side of said detecting element and an RF return bypass capacitor connected across the video transmission line to ground.
 2. In coaxial line apparatus of the type used as a termination and detector for high frequency signals, an input coaxial transmission line comprising an inner conductor and an outer conductor, means forming a junction so that said input coaxial transmission line branches into a load transmission line and a parallel transmission line, said load transmission line haviNg inner and outer conductors, a matching resistance having a first terminal connected to said inner conductor and a second terminal coupled to said outer conductor of said load transmission line for terminating the same, said parallel transmission line having an outer conductor and inner conductive means, separate from said load transmission line, said parallel transmission line inner conductive means having portions including lossy material and a high impedance detecting element, said parallel transmission line inner conductive means terminating in an inner conductor output portion coaxial with said parallel transmission line outer conductor.
 3. Coaxial line apparatus in accordance with claim 2 in which said high impedance detector comprises a crystal diode.
 4. Coaxial line apparatus in accordance with claim 3 wherein the portion of said parallel transmission line inner conductive means comprised by said crystal diode forms an angle with respect to the axis of said parallel transmission line outer conductor and wherein said parallel transmission line outer conductor has an apertured portion substantially aligned with said crystal diode whereby said crystal diode can be inserted or removed through said apertured portion.
 5. Coaxial line apparatus in accordance with claim 4 in which the angle formed by said crystal diode with respect to the axis of said parallel transmission line outer conductor is substantially a right angle.
 6. Coaxial line apparatus in accordance with claim 4 in which the portion of said lossy transmission line inner conductive means comprised by lossy material is also disposed at the same angle with respect to the axis of said lossy transmission line outer conductor as said crystal diode, said lossy material extending into said apertured portion of said outer conductive means and surrounding a portion of said crystal diode, means for adjusting the position of said lossy material with respect to said crystal diode to equalize the response of the crystal diode to the high frequency signals over a wide frequency range.
 7. Coaxial line apparatus in accordance with claim 4 wherein said lossy transmission line inner conductive means includes an impedance element connected between said crystal diode and said inner conductor output portion, said impedance element having resistance and capacitance and including a center conductor in series with a lumped resistance and lossy material forming a capacitor bead surrounding said center conductor and lumped resistance.
 8. Coaxial line apparatus in accordance with claim 7 including an additional impedance element comprising a capacitance connected in said lossy transmission line between said lumped resistance and said outer conductor, said additional impedance element cooperating with said impedance element to form an RC filter for filtering out signals on said lossy transmission line below a predetermined frequency.
 9. Coaxial line apparatus in accordance with claim 8 including a chip resistor having a relatively high resistance connected between said impedance element and said inner conductor output portion for attenuating relatively low frequency signals on said lossy transmission line.
 10. Coaxial line apparatus in accordance with claim 2 wherein said matching resistance is a rod resistance forming a continuation of said load transmission line inner conductor and wherein said load transmission line outer conductor has a plurality of discontinuous steps in the vicinity of said matching resistance approximating a tractorial taper to compensate for slow signal propagation through said matching resistance.
 11. Coaxial line apparatus in accordance with claim 3 wherein said means forming a junction includes a center conductor section having one end continuous with said input center conductor and having another end connected to said matching resistance first terminal, said center conductor section having an aperture between its ends mating with said crystal diode.
 12. Coaxial line apparatus in accoRdance with claim 11 wherein said center conductor section has undercut portions for introducing series inductance on either side of the crystal diode connection to the center conductor section in order to compensate for capacitive effects associated with said crystal diode.
 13. Coaxial line apparatus in accordance with claim 12 wherein said center conductor section undercut portions overcompensate for the crystal diode capacitance and including a threaded aperture in said load transmission line outer conductor adjacent the first terminal of said matching resistance and further including a threaded screw member disposed in said threaded aperture and threadably adjustable therein for inserting additional capacitance to bring the effective discontinuity susceptance of said junction means to zero.
 14. Coaxial line apparatus in accordance with claim 2 wherein said input transmission line outer conductor has a coaxial recess, an insulating bead mounted in said coaxial recess, said insulating bead having a central opening through which said input transmission line inner conductor passes.
 15. Coaxial line apparatus in accordance with claim 14 in which said insulating bead has its central opening counterbored to introduce inductance to compensate for the capacitance introduced by said coaxial recess in said input transmission line outer conductor. 